The subject invention pertains generally to electronic communications and specifically to communications systems entailing the transmission of digital data via modulated carrier signals through a medium polluted with harmonic signals.
The transmission of digital data by carrier signal through some medium may entail discriminating against harmonic signals which pollute the medium in order to properly detect and demodulate the carrier signal to retrieve the data. An example of such a system is the use of electric power systems as a communications channel for data which allows utility customer loads to be monitored for accounting purposes and controlled for load management from a central remote site. In addition to the broadand and impulse noise found on a power system which pose a hostile environment for communications signals, harmonic noise consisting of harmonics of the power system fundamental frequency (e.g. 60 hertz in the United States and 50 hertz in European countries) and hereinafter referred to as system harmonics must be obviated if a meaningful communication signal is to be transmitted and received using reasonable amounts of communication signal power.
In a copending patent application filed concurrently herewith entitled "Carrier Transmission Through Harmonic Polluted Medium" owned by the same assignee as the instant application, the foregoing problem is addressed by asymmetrically positioning the carrier signal between two adjacent system harmonics and employing a baud rate for the data so that all system harmonics, including the two adjacent harmonics coincide with the (sin x)/x spectral density nulls [where x=.pi.T(F-F.sub.c) with F being the frequency variable, F.sub.c the carrier frequency and T the baud period] of the modulated carrier signal in the frequency domain, thus affording efficacious discrimination against the system harmonics in the receiver detector having a matching time and frequency response. As taught therein, if greater discrimination against the system harmonics is desired, then additional minor lobes can be interposed between the carrier signal and the adjacent system harmonics, so that transmitted energy and matched receive filter sensitivity associated with lobes near the harmonics are reduced, but only at the expense of reduced data rates. In a time of rapidly increasing data communications, such a constraint may be a serious impediment to any commercially viable communications system.
Apart from benefits that may be exploited as taught in the cited copending patent application, it is advantageous to provide a means of increasing and/or optimizing data rates in a limited bandwidth, while still achieving discrimination against the system harmonics by continuing to render the spectral density nulls of all communication signals coincident with the system harmonic frequencies and providing the corresponding spectral density matched detectors.
With the foregoing in mind, it is a primary object of the present invention to provide a new and improved means for transmitting and receiving digital data modulated communication signals via a medium polluted with system harmonic signals.
It is a further object of the present invention to provide such a new and improved means which affords an increase and/or optimization of data rates within a limited bandwidth without loss in system harmonic discrimination.
The foregoing objects as well as others and the means by which they are achieved through the present invention may best be appreciated by referring to the Detailed Description of the Invention which follows together with the appended drawings.